Considerable success has been achieved thus far in this project toward the development of a nonmetallic temperature probe which uses a small optical sensor positioned at the end of a fiberoptic bundle. Important biological applications of such a nonperturbing temperature probe include the monitoring of microwave biohazards studies, microwave thawing of organs and blood, microwave-induced hyperthermia, and other experiments where conventional metallic probes perturb the fields and cause temperature errors. Both the optical etalon type of sensor and a new development, the semiconductor band edge sensor, have proven capable of temperature measurements in the laboratory with the range and accuracy required for biological studies. The semiconductor sensor is especially suited for small diameter (less than 0.25 mm) probes. Some additional effort is needed now in integrating these probes into thermometry systems which are optically and electronically stable and which have the convenience of direct readout in degrees Celsius. To improve stability, we propose to add an optical reference channel to the system to cancel optical source variations (the major cause of instability), either by separate photodetection or by optical beam chopping followed by ratioing. To get direct readout in spite of an inherently nonlinear calibration curve, we propose to use a digital read-only memory programmed with the calibration curve. These improvements are intended to make the nonperturbing thermometry system more attractive for wide-spread practical use, and will be field tested for worthiness.